In the core of a boiling water (BWR) or pressurized water (PWR) nuclear reactor, the fuel assembly components made of zirconium alloy are subjected to severe stresses which lead to their deformation. The main components affected are the mixer grids, the guide thimbles, the tubes containing the fuel pellets and the cans.
Deformation of the structural components of the assembly can cause numerous problems in use. For example, during operation, deformation of the assembly as a whole, caused essentially by deformation of the guide thimbles or of the can, can interfere with the operation of the control clusters which permit control of the reactor. Or, during operations of loading and unloading the reactor core, deformation of components such as the grids or the can increases friction and the risks of catching. This often leads the plant operator to reduce the handling speeds, thus increasing the time for which the reactor is not available. Likewise, beyond a certain threshold, deformation of the components means that they are no longer able to perform their functions completely safely and can lead the plant operator to unload the fuel assembly in question prematurely.
The stresses to which these components are subjected are substantially:                temperature;        mechanical stresses which are liable to cause creep of the components;        corrosion by the water circulating in the reactor;        hydriding;        irradiation by the neutron flux, which leads to phenomena of enlargement (free growth) and intensifies corrosion.        
The choices of the materials used to produce the structural components of the assembly and the size thereof must take into account all these stresses. Different grades of zirconium alloy have been developed for that purpose, but the choice of a material remains empirical, often resulting in the use of the same alloy for different components, such as the mixer grids and the guide thimbles (PWR) or the cans (BWR), even when they are not subjected to the same stresses.